126 SELECTION AND USE 



paid not only to its general excellence, but to its adaptability 

 to the objectives that are to be used with it. In the higher 

 departments of microscopy, the latter is a most important point, 

 but one which is too frequently neglected. It does not, how- 

 ever, come within the scope assigned to the present work, and 

 we, therefore, content ourselves with a few general hints. 



The lenses composing the eye-piece, should be of homogeneous 

 glass that is, free from air-bubbles, specks and strise, and the 

 surfaces should be well polished. These points require atten- 

 tion, because we have in our possession a microscope in which 

 though it cost enough money to be free from such defects 

 they are glaringly apparent. On looking through the eye- 

 piece at a strongly and evenly illuminated surface, the entire 

 field of view that is, the whole of the bright circle that is 

 seen, should have the light evenly diffused over its surface, 

 and the edges or border of this circle should be sharp and 

 black. 



Eye-pieces intended for first-class objectives should give a 

 large field of view; but on the other hand, if, French objectives 

 be used, the field of view should be small, otherwise the defi- 

 nition will be poor. This is a point that is frequently over- 

 looked, and we have seen very fair object-glasses condemned 

 as worthless when used with a stand and eye-piece intended 

 for objectives of an entirely different class. It is an easy thing 

 to contract the field of view, by means of a round piece of thin 

 sheet metal, having a hole of proper size in the centre. As pre- 

 viously explained, such a piece of metal is called a diaphragm, 

 and should always be well blackened. 



The magnifying power of every microscope depends upon 

 three things: The focal length of the objective, the length of 

 the body, and the eye-piece. Most microscopes are, therefore, 

 furnished with several eye-pieces, whereby the magnifying 

 power may be varied. There is, however, a limit to the extent 

 to which this may be done. The image obtained by very deep 

 eye-pieces, as they are called, is rarely satisfactory. 



The different eye-pieces are generally denoted by letters A, 

 B, C, D, etc. A being the lowest, and B, C, D, etc., successively 

 higher. Some makers use numbers 1, 2, 3, 4, etc. These 

 letters and number, are, however, entirely arbitary, in this point 



